1. Field of the Invention
The present invention relates to a wear resistant sintered alloy used in valve seat materials for automotive engines and to a manufacturing method therefor, and more particularly, relates to a developing technology of sintered alloy which may be advantageously used in valve seats in heavy duty engine such as CNG engine or diesel engine.
2. Description of the Related Art
Recently, engines for automobile are sever in operating condition toward high performance, and valve seats are required to withstand in more extreme environmental conditions than ever. For example, in LPG engines widely used in taxicabs, contacting surfaces of valves and valve seats are used in a dry condition, and they become worn more quickly than in gasoline engines. In environments having heavy sludge deposits such as in high leaded gasoline engines with high lead content, the wear is increased by the sludge when the surface pressure on the valve seat is high or when high temperature and high compression ratio is applied such as in diesel engines. When used in such severe environments, a high strength which does not cause a phenomenon of a plastic deformation is required for high wear resistance.
On the other hand, in order to adjust the valve position and valve driving timing automatically, a dynamic valve mechanism having a lash adjuster has been developed, but the problem of engine life due to wear of valve seats is not solved sufficiently, and development of a valve seat material which is excellent in wear resistance has been demanded. More recently, aside from higher performance, development of economical and inexpensive engines is equally important, and hence the sintered alloy for valve seats must have high temperature wear resistance and high strength without requiring additional mechanisms such as a lash adjuster.
As such a sintered alloy for valve seats, Japanese Patent Publication No. S59-037343 (patent reference 1) (U.S. Pat. Nos. 4,422,875, 4,552,590) proposes to disperse Co—Mo—Si hard phase in a dappled matrix of a Fe—Co alloy and a Fe—Cr alloy. Japanese Patent Publication No. H05-0955593 (patent reference 2) proposes to disperse Co—Mo—Si hard phase in a Fe—Co alloy matrix. Japanese Patent Publication No. H07-098985 (patent reference 3) (U.S. Pat. No. 4,919,719) proposes to disperse Co—Mo—Si hard phase in a matrix adding Ni to a Fe—Co alloy. Japanese Laid-open Patent No. H02-163351(patent reference 4) proposes an Fe matrix alloy dispersing Co—Mo—Si hard phase.
Hard phase in the alloys disposed in these patent references 1 to 4 have a Mo content of 40 mass % or less, but sintered alloys containing these hard phase have considerable high temperature wear resistance and high strength. However, sintered alloys having wear resistance and high strength in high temperature are desired. For example, an improved invention discloses alloy powder for forming wear resistant hard phase consisted of Si: 1.0 to 12 mass %, Mo: 20 to 50 mass %, Mn: 0.5 to 5.0 mass %, and balance of at least one of Fe, Ni, and Co, and inevitable impurities (see Japanese Laid-open Patent No. 2002-356704, patent reference 5).
Thus, according to the demand of the times, various sintered materials with excellent wear resistance which may be favorably used as valve seat materials have been proposed. However, in recent CNG engines, heavy duty diesel engines for high output, the load on the valve seat material is much higher due to metal contact, and there is a keen desire to develop materials having high wear resistance in all severe environments.